This invention relates generally as indicated to a tire testing machine and, more particularly having a zero stiffness arrangement for loading a tire/wheel assembly onto a simulated road surface.
To predict, correct or otherwise test the performance of a vehicle, it is common in the tire manufacturing arts to formulate a complete vehicle dynamic model. Because the tire/wheel assembly is the only connection between a vehicle and a road, a tire/wheel assembly can strongly influence the dynamics of the whole vehicle system. Accordingly, it is important to know natural frequencies, mode shapes and modal damping of the tire/wheel assembly that will be used on a vehicle for verification purposes, such as the verification of Finite Element Analysis models. To this end, the tire/wheel assembly will usually be tested independently of the rest of the vehicle model to obtain the necessary verification.
Since non-rotating and rotating tires have different dynamic properties, a tire testing machine is usually used during testing procedures. Such a tire testing machine can comprise, for example, a movable simulated road surface, an axle including a bearing surface around which a tire/wheel assembly can rotate, and a frame which loads the tire/wheel assembly into rolling engagement with the simulated road surface. The simulated road surface can reside on, for example, the outer circumferential surface of a wheel. In that manner, one can test a tire rolling on a simulated road surface in an otherwise stationary tire testing machine.
What the art needs is a tire testing machine that minimizes variables of simulation to permit Finite Element Analysis and other verifications to be conducted in a manner that essentially duplicates the real tire/wheel assembly on an actual vehicle on an actual road surface.
More particularly, what the art needs is a tire testing machine which allows testing of a tire under with zero stiffness boundary conditions relative to the simulated road surface. Without zero stiffness boundary conditions, the contributions of the machine to measured frequencies must be accounted for in order to determine the natural frequency of the tire. Such accounting is difficult to do in an accurate manner, especially when low frequency (under 20 HZ) models are being developed. A zero stiffness boundary condition can be easily and accurately modeled in Finite Element Analysis and also enables the lower tire/wheel natural frequencies to be measured in all three principal directions.
The present invention provides a tire testing machine having a frame which enables a tire/wheel assembly to be loaded and rolled on a simulated road surface with nearly zero stiffness boundary conditions. With particular reference to a curved simulated road surface (e.g., a road wheel), pre-loaders are provided to enable the tire/wheel assembly to be statically loaded onto the curved surface in a stable manner.
More particularly, the present invention provides a tire testing machine comprising a movable simulated road surface; an axle including a bearing surface around which a tire/wheel assembly can rotate; and a frame. The frame supports the axle in a non-rotatable but vertically floatable manner relative to the simulated road surface whereby the tire can be loaded in rolling engagement against the simulated road surface. To this end, the frame comprises tie members which extend from the axle to a stationary support in a geometrically sturdy arrangement and non-damping piston members which are interposed between the tie members and the stationary support. The simulated road surface can comprise the outer circumferential surface of a road wheel. xe2x80x9cGeometrically sturdyxe2x80x9d means that the connection of frame and tie members results in a substantially inflexible structure based on the geometry of the connections to dynamically isolate the road/wheel assembly from the remainder of the tire testing machine. Preferably, the geometrically sturdy arrangement is a triangular arrangement because of the minimal number of components. However, for a variety of reasons, it can be desirable to use a higher number of components in another geometrically sturdy arrangement such as multiple triangular, pentagonal, hexagonal, and octagonal arrangements, or combinations thereof. Also, a linear alignment of tie members may provide, in certain circumstances, a geometrically sturdy arrangement.
The piston members can be adjustable to initially set the load of the tire/wheel assembly (or preload the tire) against the movable simulated road surface. These piston members can comprise air pistons and more preferably frictionless zero-stiffness air pistons. The tie members can be connected to the air pistons via slides which are cushioned with air bearings to eliminate nearly all damping contribution from the slides. The preferred geometrical arrangement of the tie members is an isosceles triangular arrangement.
To test a tire/wheel assembly with the tire testing machine of the present invention, the tire/wheel assembly is rotatably mounted on the bearing surface of the axle and the tire is loaded into rolling engagement with the simulated road surface. The simulated road surface is then moved to rotate the tire/wheel assembly whereby test data can be obtained under substantially zero stiffness boundary conditions such as measurements of the lower tire/wheel natural frequencies in the three principal directions. This test data can then be modeled in various finite element analyses known to and used by those skilled in the art.
The present invention provides these and other features hereinafter fully described and particularly pointed out in the claims. The following description and drawings set forth, in detail, a certain illustrative embodiment of the invention. This embodiment is indicative, however, of but one of the various ways in which the principles of the invention can be employed.